Stepper motors are widely used in applications requiring accurate position control and compatibility with digital control systems. Electrical pulses of prescribed pulse width and amplitude advance the motor by a predetermined distance for each pulse. One standard stepper motor type requires 200 pulses for one complete revolution, thus providing 1.8.degree. of revolution per pulse. When a pulse train is supplied to the stepper motor, it rotates continuously at a rate determined by the pulse repetition frequency. One advantage of a stepper motor is that its position can be determined by counting the pulses supplied to it, assuming that no slippage occurs.
One drawback of stepper motors is that acceleration and deceleration times have been relatively long due in part to the available torque characteristic of stepper motors. During acceleration and deceleration, the stepper motor is accelerated and decelerated in accordance with a predetermined velocity profile, or velocity variation as a function of time. Linear ramping of stepper motor velocity is well known in the art and requires a constant torque. Exponential velocity profiles have also been utilized. See, for example, Kuo, Step Motors and Control Systems, SRL Publishing Co., 1979, pp. 135-143. A linear velocity increase followed by an exponential increase is disclosed in U.S. Pat. No. 4,496,891 issued Jan. 29, 1985 to Kobayashi. Any velocity profile is limited by the torque available from the motor. Stepper motors have a torque versus speed characteristic which is maximum at low speed and decreases with increasing speed. If the available torque curve is exceeded during acceleration or deceleration, the motor will stall. As a result, it is necessary to utilize a velocity profile wherein the required torque stays below the available torque by a given safety margin. In the linear case, the required torque is constant and is limited by the available torque at the highest operating speed. Consequently, the relatively high torque available at low speed is not utilized. It is desirable to provide a velocity profile which efficiently utilizes the available torque of the stepper motor so that optimum acceleration and deceleration characteristics can be achieved.
A further drawback of existing stepper motor controllers relates to the difficulty in implementing different maximum motor velocities and acceleration/deceleration times. Prior art controllers have usually utilized analog circuitry such as constant current charging of a capacitor for generating a linear velocity profile and constant voltage charging of a capacitor for generating an exponential velocity profile. Accurate programming of such circuits entails substantial complexity and expense. It is desirable to provide a stepper motor controller in which the velocity profile during acceleration and deceleration can be easily programmed to accommodate different motor characteristics, different applications, and different operating parameters. Stepper motor controllers utilizing microprocessors are disclosed in U.S. Pat. Nos. 4,496,891, Kobayashi; 4,417,188, Makabe et al; 4,156,170, Strunc and 4,568,866, Floro et al. However, it is believed that none of the disclosed systems fully utilize the available stepper motor torque to obtain high acceleration and deceleration rates without stalling or missing steps.
It is a general object of the present invention to provide improved methods and apparatus for stepper motor control.
It is another object of the present invention to provide methods and apparatus for stepper motor control utilizing a parabolic velocity profile as a function of time.
It is yet another object of the present invention to provide methods and apparatus for stepper motor control wherein a parabolic velocity profile is determined from selected values of start/stop velocity, maximum velocity and time to reach maximum velocity, and is stored in a random access memory.
It is still another object of the present invention to provide methods and apparatus for stepper motor control which are easily programmed to accommodate different motor characteristics, different applications and different operating parameters.